The Homologous Recombination Core will make available the technology of targeted gene disruption of homologous recombination. 1. Maintain a full complement of genomic libraries, selectable markers, and bacterial and phage vectors required for gene modification in mouse embryonic stem (ES) cells and other rodent and human cultured cell lines. 2. Provide consultation services regarding all steps in the gene modification protocols including vector design, selection strategies and recombinant identification. 3. Provide an ES cell manipulation service. 4. Develop new technology in four areas: (a) isolation of new ES cell lines (b) adaptation of novel cell/embryo methods (c) development of gene targeting in non-ES lines (d) development of a conditional-mutant system applicable to the adult mouse